NHS Blood and Transplant has triggered an 'amber alert' on its national stocks.

The NHS aims to hold above six days of blood stocks, but levels are currently predicted to fall below two days, the threshold for the alert.

This means, from October 12, hospitals have been asked to put management plans in place to protect blood stocks, and there is a possibility non-urgent elective surgeries could be delayed so blood is prioritised for patients who need it most.

O blood groups - positive or negative - are needed in particular.

The NHS branch is urging donors to fill appointments at permanent donor centres which have extended hours and greater capacity.

There are 400 appointments available at Stratford donor centre and 130 at Tooting donor centre this week. If there are no appointments straight away, donors are encouraged to book for the coming weeks.

Wendy Clark, interim chief executive of NHS Blood and Transplant, said: “Asking hospitals to limit their use of blood is not a step we take lightly. This is a vital measure to protect patients who need blood the most.

“Patients are our focus. I sincerely apologise to those patients who may see their surgery postponed because of this.

“With the support of hospitals and the measures we are taking to scale up collection capacity, we hope to be able to build stocks back to a more sustainable footing."

She added: “We cannot do this without our amazing donors. If you are O positive or O negative in particular, please make an appointment to give blood as soon as you can. If you already have an appointment, please keep it.”

Hospitals will make individual decisions but the surgeries that could be postponed will be non-urgent, such as hip replacements.

Surgeries which will continue include urgent, emergency, trauma, cancer and transplant, alongside blood transfusions to treat those with long term conditions.

Professor Cheng-Hock Toh, chair of the National Blood Transfusion Committee, said: "I know that all hospital transfusion services, up and down the country, are working flat out to ensure that blood will be available for emergencies and urgent surgeries."

The amber alert will last initially for four weeks.

Donors in the O blood group are asked to call 0300 123 2323, download the NHS Give Blood app or visit www.blood.co.uk/ to book appointments.